In 1920s, Yagi Hidetsugu and Uda Shintarou of Tohoku University in Japan invented an aerial that is later named as “Yagi-Uda aerial”, and also named as “Yagi Aerial” for short. Yagi aerial is still used a lot until now, because it has good directionality and can provide a better gain than dipole aerial, and the effect of long distance communication is especially good to do direction-finding with Yagi aerial. A typical Yagi Aerial should have three pairs of dipoles, and the three pairs of dipoles are parallel in a same plane. The pair of dipoles connected to the cable is called as active dipole, or called as main dipole, which is the middle one of the three pair of dipoles. The pair of dipoles that is a little longer than the active dipole is called as reflector, which is located at one side of the active dipole and has the function of weakening the radio wave received from the side or sent out from the side; the pair of dipoles that is a little shorter than the active dipole is called as director, which is located at the other side of the active dipole and has the function of enhancing the radio wave received from the other side or sent out from the other side; wherein, the quantity of the director may be plural; a support tube is located at the middle of the reflector, the active dipole, and the director, which is vertical to them and has the function of supporting and fixing them.
In modern life and engineering, varied dipole forms can be used as the active dipole of a Yagi aerial; wherein, the most common active dipole is half wavelength symmetrical dipole and half wavelength folded dipole; moreover, in an engineering aerial, for being restricted by the impedance characteristic of the two dipole types themselves, the half wavelength folded dipole is most common to be used as the active dipole of a Yagi aerial. In a Yagi aerial that uses the half wavelength folded dipole as the active dipole, the feeding structure mainly utilizes the structure of the Yagi aerial itself; for example, in the compact feeding method of penetrating type, the cable is penetrated via the support tube, and through the elbow pipe of the folded dipole strides over the feeding point, to connect the inner conductor of the cable to the opposite folded dipole; for being restricted by the structure, if a comparatively higher gain and front-to-rear ratio are needed, this kind of Yagi aerial must be made big enough, which will lead to the increasing of the production cost, the occupation space, etc. The half wavelength symmetrical dipole is also called as half wavelength dipole; for being restricted by the impedance characteristic of itself, the half wavelength dipole is usually used in a civil homemade Yagi aerial, which can not achieve the parameter requirement of an engineering aerial.